The present invention provides both a method of mixing and separating two immiscible liquids of different densities, and an apparatus associated with said method. Such apparatus may be referred to as a mixer/separator installation or as a mixer/decanter device. Liquid/liquid extraction is performed therein.
The preferred embodiment of the present invention lies in the context of mixing/separating two liquids as implemented in a mixer/decanter device of the reverse liquid flow type [the decanter being fed from the end of the decanter (its "feed end") that is farthest from the mixer] and with a superposed spout (said mixer and decanter are connected together by a spout disposed above said decanter).
The technical field to which the invention relates is specified a little further on.
The mixing and separating method consists in producing a dispersion of two immiscible liquids, one within the other, that is sufficiently fine to facilitate interchanges between the liquids, and then separating the two immiscible liquids again by decanting (with such separation by decanting being possible providing said two liquids do not have the same density). Generally, one of the liquids is constituted by an impure solution of a given substance while the other liquid is an organic solvent serving either as a solvent that is specific to the substance under consideration or else as a dilution medium for an ion exchanger that is specific to the substance under consideration. The complete installation generally comprises an extraction bank comprising a plurality of mixers/decanters in juxtaposition within which the impure solution of the substance under consideration flows as a counterflow to the organic liquid, followed by a re-extraction bank of identical type in which the organic liquid flows as a counterflow to a pure solution that collects the substance, thereby enabling said organic liquid to be used in a closed circuit.
Each mixer/decanter in the bank essentially comprises a mixer receiving the two liquids, generally via its base, and producing the sought mixture (dispersion) by stirring, followed by a decanter vessel in which said mixture flows slowly and separates progressively, specifically because the liquids are immiscible and the dispersion is not stable. For a given flow rate, it is clear that the size of each mixer depends on the transit time the dispersion requires before going to the separator, which time depends on exchange efficiency, which efficiency increases with increasing fineness of the dispersion. Similarly, the size of each decanter vessel depends on the transit time of the dispersion passing through it, which time is a function of the coalescence speed of the dispersion which, on the contrary, decreases with increasing fineness of the dispersion. Consequently there is always an optimum size for the droplets in the dispersion that provides minimum overall bulk and thus a minimum price for the installation as a whole.
The liquids and mixtures of liquids are generally moved through the installation by means of pumps or pump/stirrers.
Mixing/separating methods and the associated apparatuses are described, in particular, in documents U.S. Pat. No. 4,235,602 and EP-A-0 252 785.
The apparatus described in patent U.S. Pat. No. 4,235,602 is a mixer/decanter of the reverse liquid flow and superposed spout type.
The apparatus described in application EP-A-0 252 785 is a mixer/decanter of the reverse liquid flow and submerged spout type.
To facilitate both the following description of the present invention and also understanding thereof, reference is now made to accompanying FIG. 1 for describing in greater detail the prior art that is closest to the present invention. The present invention comprises an improvement over that prior art. Nevertheless the improvement is not limited solely to that context.
A mixer/decanter of said prior art is shown diagrammatically in FIG. 1. It comprises a mixer/decanter of the type described in patent U.S. Pat. No. 235,602, i.e. of the reverse liquid flow and superposed spout type. Said mixer/decanter advantageously constitutes one stage of an extraction/re-extraction bank.
In FIG. 1, the following are shown:
the mixer 1 which includes, in particular, separate liquid inlets 2 and 3; PA1 a pump 5 which raises to an appropriate level the mixture (or the dispersion) formed in said mixer 1 by stirring means 4; PA1 a spout 6 which transports (or conveys) the liquid mixture leaving the pump 5 to the inlet to the decanter vessel 7 in which the two liquids flow at low speed and separate progressively so as to give rise to a light phase (generally organic) which is removed at 8 together with a heavy phase (generally aqueous) which is removed at 9; and PA1 a phase boundary regulator 12. PA1 feeding a mixer with said two liquids so as to fill the mixer up to a given level; PA1 stirring within said mixer so as to produce a dispersion of said two liquids; PA1 conveying said dispersion to decanting apparatus; this being done under conditions where said dispersion pre-separates into two fractions, each enriched with a respective one of said two liquids; and PA1 separating said two liquids within said decanting apparatus. PA1 to fix the volume and/or the shape of each of the separation units independently; and thus PA1 to advantage decanting in one of the two compartments of compartmented decanting apparatus. Under such circumstances, said apparatus is generally not partitioned into two equal portions. PA1 separating out traces of the lighter liquid (generally an organic phase) contained in the stream of the heavier liquid (generally an aqueous phase) by placing an appropriate coalescence promoter therein (an oil-absorbing substance). If bubbling means and possibly also dross-collection means are already present within said stream of heavier liquid, then said coalescence promoter may be superfluous. In any event, if used, it is naturally located downstream from said bubbling means and said dross collection means; and PA1 separating out traces of the heavier liquid (generally an aqueous solution) from the stream of lighter liquid (generally an organic phase), by placing therein an appropriate (water-absorbing) coalescence promoter. PA1 the dimensions or volumes and/or shapes of the decanting units or compartments; and PA1 the phase boundary thresholds within said units or compartments. PA1 in the decanting compartment or unit to which there is directed the pre-separated fraction that is enriched in the heavier liquid, i.e. the compartment or unit having the thick bottom stream, means for implementing flotation by bubbling gas (e.g. air or inert gas). Said means are advantageously associated, downstream, with means for recovering dross entrained by said flotation. It is also possible to provide a coalescer downstream from said means for implementing flotation and recovering dross, which coalescer is therefore not dirtied with dross. Whether it is appropriate to make use of such a coalescer constitutes a judgment to be performed by the person skilled in the art. Flotation can itself make such a coalescer entirely superfluous; PA1 in one or both of the decanting compartments or units, means to promote coalescence within the thicker stream (i.e. the stream constituted by the majority phase). Said means must clearly be adapted to the phase in which they operate. They should have surface tension characteristics adapted to the nature of the minority phase that is to be eliminated from the majority phase. As mentioned above, the use of such means may turn out to be superfluous in a thick stream that is rich in heavier liquid within which bubbling has already been implemented. Nevertheless, in this context, such use need not be excluded, and in any event it may be highly advantageous in any such a stream (heavier liquid, thick) within which no bubbling has been performed. PA1 either be united with the main flow from the other compartment or unit (providing the drives are compatible); PA1 or be recycled to the mixer; PA1 or else be removed from the circuit for special treatment.
The pump 5 shown is a cone type pump (as described in application FR-A-2 343 905 and also in patent U.S. Pat. No. 4,235,602). Use thereof is particularly advantageous insofar as it does not give rise to any additional stirring and on the contrary, does give rise within itself to a certain amount of coalescence, beginning with the effect of the centrifugal force on the conical stream that flows without turbulence.
In such apparatus, the liquids at the end of the spout 6 furthest from the mixer 1, i.e. the end feeding the decanter vessel 7 have already undergone pre-separation or pre-decanting (beginning in the cone pump 5 and developing during slow progress of the liquids along the spout 6).
To benefit from this pre-separation and to avoid or at the very least limit re-mixing of said pre-separated liquids during transfer via the spout 6 to the decanter vessel 7, means are advantageously provided at said end of the spout 6 (such as independent wiers at different levels across the entire width of said spout 6, . . . ) and/or at the inlet to the decanter vessel 7. FIG. 1 shows a grid 13 extending across the entire width and the full height of said tank 7. This grid 13 constitutes a distributor within said tank 7 and separates a compartment 14 at the end thereof that is furthest from the mixer 1. This grid 13 is made up of a plurality of parallel wall elements that are substantially horizontal or that are preferably inclined in the direction that encourages coalescence and retention of the discontinuous phase, with inclination depending on whether the said discontinuous phase is denser or less dense than the continuous phase.